1. Field of the Invention
The present invention relates to the forming of a photolithography mask for the manufacturing of a microstructure by grey level etching of a resist layer. It more specifically aims at a method for determining the mask pattern based on a desired microstructure profile.
The examples and embodiments discussed hereafter more specifically relate to masks intended for the manufacturing of a microlens coating the surface of exposure to light of an image sensor pixel. The present invention however more generally applies to any type of microstructure that may be formed by grey level etching of a resist layer, for example micromechanical structures.
2. Discussion of Prior Art
An image sensor is essentially formed of a pixel array formed inside and on top of a semiconductor substrate. At the surface of each pixel, a microlens is provided to concentrate the received light intensity towards a photosensitive area of the pixel.
To simultaneously form all the microlenses of a sensor, the use of a grey level resist etch method has been provided. Such a method especially comprises, in a first step, depositing a resist layer on the surface of exposure to light of the sensor. The resist is exposed through a grey level photolithography mask. The intensity of the irradiation received by the resin varies in space according to the position in the mask. In another step, the resist is developed. The sensitivity of the resist to development is proportional to the intensity of the irradiation received during the exposure. The amount of resist remaining after the development is thus inversely proportional to the grey level of the mask. One can thus, by defining an adapted mask pattern, “carve” microlenses of different shapes in the resin layer. An additional stabilizing anneal step may be provided after the development.
A benefit of such a method is that it allows a lot of flexibility as to the shape of the microlenses which may be formed. In particular, it is possible to form on a same sensor, in a single photolithography step, microlenses of different shapes. As an example, French patent application FR2945666 describes an image sensor in which each pixel, other than the central pixel (s) of the array, comprises an asymmetrical microlens topping in vertical projection the substrate portion associated with the pixel. The shape and the optical axis of each microlens are selected according to the pixel position in the array, so that the received light rays converge towards the photosensitive area of the pixel. Thus, for each pixel, the shape of the microlens is adapted to the average angle of incidence of the light rays received by the pixel, which improves the sensitivity with respect to sensors in which all microlenses are identical.
FIG. 1 is a top view schematically showing an example of a photolithography mask 1 for the grey level etching of a microlens in a resist layer.
Mask 1 is essentially formed of a transparent plate 1a, for example, made of glass or of quartz, having opaque areas 1b formed at its surface, for example, in the form of chromium pads or islands or of any other suitable opaque material.
Mask 1 comprises a plurality of elementary cells 3, each corresponding to a portion of the mask. In this example, cells 3 are, in top view, square and arranged in an array of 7 rows by 7 columns side by side. The division into elementary cells is symbolized in the drawing by a grid in dotted lines. Opaque areas 1b have, in top view, dimensions smaller than the dimensions of cells 3. Each cell 3 comprises an area 1b arranged in its central portion. In this example, each area 1b has the shape of a square having its center coinciding with the center of the corresponding cell 3.
In a given cell 3, the grey level corresponds to the ratio between the surface area taken up by opaque area 1b and the total cell surface area. The higher the grey level of the cell, the smaller the thickness of etched resin. Conversely, the lower the grey level of the cell, the greater the thickness of etched resin. By varying the size of pads 1b, the grey level of each cell can be varied, to thus define various etch levels. The pattern of the mask shown in FIG. 1 corresponds to the forming of a symmetrical microlens.
To define the mask pattern, the three-dimensional profile of the microstructure which is desired to be formed is divided into elementary areas corresponding, in top view, to cells 3. For each elementary area, the average resin height necessary to achieve the desired profile is determined. A curve of correspondence between the irradiation intensity received by the resin and the resin height remaining after development enables to define the opaque surface area 1b to be provided in each cell 3 of the mask.
In practice, for the resin exposure, mask 1 may be placed directly in contact with the resin layer, or be suspended above the resin by means of wedges, or again be used in an optical system in projection. The step of elementary cell array 3 is selected so that, taking diffraction phenomena into account, the space variations of the irradiation intensity received by the resin are progressive. This enables to form a microstructure with a curved surface, for example, a microlens, from a mask only comprising black (opaque) and white (transparent) areas.
In many cases, and in particular for microstructures with curved surfaces, it is not possible to obtain an exact matching between the ideal desired profile and the profile really printed into the resin. The accuracy with which any desired profile can effectively be obtained is linked to the resolution of elementary cell array 3 of the mask. Increasing the resolution would amount to decreasing the array step, which is generally impossible due to the above-discussed optical constraints, and due to the limited resolution of mask manufacturing tools.
FIG. 2 is a cross-section view schematically illustrating profiles 21R and 21I of a microlens coating the surface of exposure to light of an image sensor pixel 20. Profile 21R corresponds to a microlens obtained by grey level etching of a resist layer, by using mask 1 of FIG. 1 for the resist exposure. Profile 21I corresponds to the ideal target profile, that is, to the profile which would really be desired to be obtained, and which has been used as a basis for the determination of the pattern of mask 1. It can be observed that real profile 21R does not faithfully coincide with profile 21I.
It would be desirable to be able to form microstructures having any type of profile, by grey level etching of a resist layer, with a greater accuracy than with current methods.